Production was halted for weeks after Walter Matthau had a heart attack. He had slimmed from 190 to 160 pounds by the time filming was completed, and had to wear a heavy black coat to conceal the weight loss.
For the scene in which he motors through the city searching for Ron Rich (aka "Boom Boom"), Jack Lemmon also served as a driver, cinematographer and gaffer.
Marked the first pairing of Jack Lemmon and Walter Matthau, who subsequently worked together on 11 additional films (including Kotch, in which Lemmon directed Matthau).
This was director Billy Wilder's second film in a row in which one of his lead actors suffered a heart attack. In preceding film, 1964's Kiss Me, Stupid, Peter Sellers' health problem forced Wilder to replace him with Ray Walston. In Fortune Cookie, Walter Matthau suffered attack midway through production but shooting was postponed until he recovered; his drastic weight loss from scene to scene is noticeable.
Lisa discovered the piano hidden in her folk's garage at the age of four. One of Lisa's fondest early memories is sharing a piano with fellow actor Jack Lemmon during the 1965-66 filming of the movie "The Fortune Cookie." (Lisa played the little girl on the skateboard in an early hospital scene.)